As known to those skilled in the art, centralisers are used in the oil, gas & water well drilling industries to center a tubular member (hereinafter referred to as a “tubular”) within a borehole or inside a previously installed larger tubular member.
Such tubulars are generally constructed in handleable lengths e.g. 12 meters, each length being externally male threaded at both ends. The lengths are assembled together using short female threaded couplings. The assembly of the tubulars to a predetermined total length is referred to as a string.
When the string is disposed in a borehole or existing tubular, it is desirable to position the string substantially centrally within the borehole or existing tubular thereby forming a substantially annular passageway around the tubular of concern. This enables passage of material such as fluids, cement slurries in the space around the tubular. Under some circumstances substantial centrality is imperative.
To try to achieve this condition, centralisers are disposed at selected intervals along the length of the string. Retention of the centralisers in a desired position may be achieved in restricting axial movement by the use of a so-called “stop collar” being a ring grippingly secured to the tubular.
The state of the art embraces solid and spring centralisers. Solid (or rigid) centralisers are commonly cast products of a fixed dimensional construction with an undersize external diameter to allow passage through the borehole. Spring centralisers are of a flexible external diameter aimed at making contact with the borehole wall at all times while being capable of flexing to accommodate obstructions or dimensional changes within the borehole.
Solid centralisers have an internal diameter with clearance to fit onto a tubular and an external diameter selected to pass into the borehole of concern. Given the axial variation of diameter of the borehole it is clear that solid centralisers cannot adequately support the tubular in a central position. Equally being solid, a solid centraliser risks jamming within the borehole.
Spring centralisers may overcome these problems. The current design is a number of hardened and tempered leaf springs, also referred to in the art as bows, located radially around and affixed to low carbon steel end bands at both ends.
However spring centralisers currently in use exhibit difficulties with under modern conditions such as depth of well, angular deviation profile and extended horizontal reach into the hydrocarbon producing strata. As a result they may be made with an oversize outer diameter to create a pre-load effect that gives an acceptable deflection versus load characteristic: however this may create undesirable insertion forces. This in turn, together with multi-part construction gives rise to the possibility of disintegration.
Known methods of securing together of the parts of the conventional centraliser include welding and mechanical interlocking of the leaf springs to the end bands—both methods of construction detract from the maximum possible load/deflection performance.
The multiple parts used to construct conventional centralisers e.g. a split and hinged variety of a more common size variant consists of fourteen individual parts, each part being at risk of breaking off and falling into the well bore.
There is thus a long felt want for a practical one-piece centraliser.
U.S. Pat. No. 3,312,285 (Solum) contains a disclosure of a one-piece centraliser consisting of two collars spaced by bows (staves) which are outwardly curved and serve to centralize a tubular member. The Patent further discloses a manufacturing technique for such a centraliser.
The manufacturing method consists of cutting a blank from a sheet of metal material by cutting or punching. The material is said to be a steel selected from a group including “plain carbon steels with a relatively high carbon content or alloy steels with a medium carbon content”. The Patent specifically envisages the use of “grades of steel . . . which are unsatisfactory for construction of centralisers using conventional methods due to such factors as the need for welding the spring bows to the end collars”. It is understood that such materials are spring, non-ductile, steels.
The manufacturing method requires the blank to be placed on a forming die having a semi-cylindrical cavity, followed by application of a press tool to form the blank into a U-shape and in turn followed by the application of an inverse die to form a “long cylinder”.
The blank is then supported at one end and the other end urged towards the one end to provide outwardly-bowed staves as required.
Finally the abutting ends of the blank are welded together by arc-welding to create a generally cylindrical centraliser.
The centraliser is then heat-treated to obtain the desired hardness.
Experiments have revealed a number of deficiencies in the technique described in U.S. Pat. No. 3,312,285. Indeed the disclosure of the U.S. patent is not believed to provide a practical method for manufacturing a centraliser. Further a device which is manufactured from material to which the method of the Patent can be applied is not believed to have the desired properties of a practical centraliser.
Firstly it is noted that the use of a cold-forming dual die system of the type disclosed in the Patent upon a spring steel would not result in a cylindrical blank. Rather, the ends of the blank which were brought into abutment by the die, would spring apart once the die were removed. It would, therefore, be necessary either to perform the forming step as a hot forming process or alternatively to physically restrain the blank in its cylindrical state. The latter technique would not permit the outward-bowing step as disclosed in the patent.
Forming the blank into a generally cylindrical body by the die technique disclosed has been found to give rise to curved end collar portions. However, the intermediate bow portions, which are separated by longitudinal apertures, do not confirm to the curved profile of the collar portions due to the presence of the apertures. The bow portions, therefore, tend to form flats, or curves of relatively unpredictable curvatures.
During longitudinal bow-forming pressure, the bows neither form uniformly nor predictably. Furthermore, unless hot forming is used the tolerances in the bows are unacceptable. Moreover, as the material used is a spring steel, it is necessary to over-bend the bows and it is not possible to determine consistently how far to over bend the bows to give rise to a desired final form.
On the basis of the experiments performed, it has been found that a centraliser in accordance with U.S. Pat. No. 3,312,285 requires the use of hot forming. This in turn means the use of expensive high temperature form tools with the resultant high tooling attrition. At least two and maybe three heating steps are required for forming followed by a heating/quenching phase to the required hardness. Then a further heating to temper stage of around 450 degrees centigrade is required.
Apart from the high cost of hot forming in this way, there is the risk of growth of grain within the crystalline structure of the material, which would give rise to weakness and the risk of breakage. Further, each of the heating steps is likely to give rise to distortion, which reduces the yield and increases the cost.
It is known that the form of the bows is desirably parabolic in the longitudinal direction. The technique disclosed in U.S. Pat. No. 3,312,285 makes this form difficult to attain on a consistent basis. The arc-welding step requires pre-heating and a slow post-weld cooling.
It is therefore believed that the product and method of the U.S. patent is impractical. If conventional ductile formable materials were used, the method would be capable of putting into effect, but the resultant product would not have the properties required of a centraliser.
It is understood that products in accordance with U.S. Pat. No. 3,312,285 are not on the market.